The present invention relates to processes and systems for liquefying natural gas. In one aspect the invention relates to such processes and systems wherein a common separator (i.e. flash tank) and vapor compressor are used by multiple trains within the system to recover vapor both for cooling and for use as a fuel gas.
Various terms are defined in the following specification. For convenience, a Glossary of terms is provided herein, immediately preceding the claims.
Large volumes of natural gas (i.e. primarily methane) are located in remote areas of the world. This gas has significant value if it can be economically transported to market. Where gas reserves are located in reasonable proximity to a market and the terrain between the two locations permits, the gas is typically produced and then transported to market through submerged and/or land-based pipelines. However, when gas is produced in locations where laying a pipeline is infeasible or economically prohibitive, other techniques must be used for getting this gas to market.
A commonly used technique for non-pipeline transport of gas involves liquefying the gas at or near the production site and then transporting the liquefied natural gas to market in specially-designed storage tanks aboard transport vessels. The natural gas is cooled and condensed to a liquid state to produce liquefied natural gas at substantially atmospheric pressure and at temperatures of about xe2x88x92162xc2x0 C. (xe2x88x92260xc2x0 F.) (xe2x80x9cLNGxe2x80x9d), thereby significantly increasing the amount of gas which can be stored in a particular storage tank. Once an LNG transport vessel reaches its destination, the LNG is typically off-loaded into other storage tanks from which the LNG can then be revaporized as needed and transported as a gas to end users through pipelines or the like.
As will be understood by those skilled in the art, plants used to liquefy natural gas are typically built in stages as the supply of feed gas, i.e. natural gas, and the quantity of gas contracted for sale, increase. Each stage normally consists of a separate, stand-alone unit, commonly called a train, which, in turn, is comprised of all of the individual components necessary to liquefy a stream of feed gas into LNG and send it on to storage. As used hereinafter, the term xe2x80x9cstand-alone trainxe2x80x9d means a unit comprised of all of the individual components necessary to liquefy a stream of feed gas into LNG and send it on to storage. As the supply of feed gas to the plant exceeds the capacity of one stand-alone train, additional stand-alone trains are installed in the plant, as needed, to handle increasing LNG production.
In typical LNG plants, each stand-alone train includes at least a cryogenic heat exchange system for cooling the gas to a cryogenic temperature, a separator (i.e. a xe2x80x9cflash tankxe2x80x9d), a xe2x80x9creject gasxe2x80x9d heat exchanger, and a fuel gas compressor. As used herein, a xe2x80x9ccryogenic temperaturexe2x80x9d includes any temperature of about xe2x88x9240xc2x0 C. (xe2x88x9240xc2x0 F.) and lower. LNG is typically stored at substantially atmospheric pressure and at temperatures of about xe2x88x92162xc2x0 C. (xe2x88x92260xc2x0 F.). To reduce the pressure of feed gas during liquefaction, it is typically passed from the cryogenic heat exchanger system across an expansion valve or hydraulic turbine in a stand-alone train (i.e. xe2x80x9cflashedxe2x80x9d) before it is passed into the separator (i.e. the flash tank). As the pressure of the cooled feed gas is reduced to produce LNG at substantially ambient pressure, some of the gas flashes and becomes vapor. LNG is removed from the flash tank and is pumped from its respective stand-alone train on to a storage tank for further handling.
Vapor (i.e. reject gas) is removed from the flash tank and is warmed in the reject gas heat exchanger by exchanging heat with the incoming feed gas and/or the refrigerant(s) used in liquefying the feed gas. The warmed gas is then passed to the fuel gas compressor in the stand-alone train to increase its pressure before the gas is passed on for use as fuel gas within the plant. It can be seen that by recovering the vapor from the flash tank and using it both for cooling within the stand-alone train and ultimately for fuel, the efficiency of the overall liquefaction process is significantly improved.
In typical LNG plants, the stand-alone trains of the liquefying process are all located near each other within the LNG plant area which, in turn, is typically located a significant distance, e.g. several kilometers, from the LNG storage tanks. During storage, heat from the surrounding environment, which inherently leaks into the LNG storage tanks, causes some of the stored LNG to vaporize resulting in xe2x80x9cboil-off gasxe2x80x9d within the tanks. Additional storage tank boil-off gas is created by: (i) energy input to the LNG by the rundown pumps that provide sufficient pressure to effect LNG transfer from the flash tank to the storage tank; (ii) heat leak through the insulation on the LNG rundown line; (iii) heat leak through the insulation on the LNG loading and recirculation line; and (iv) energy input to the stored LNG by the recirculation pump(s). While this boil-off gas is typically recovered and compressed for use as fuel gas, any attempts to also use this boil-off gas for heat exchange (i.e. cooling) within the gas liquefying process is usually uneconomical due to the distance this gas must travel between a respective storage tank and a respective stand-alone train within the plant area.
It would be desirable if certain functions, which are normally carried out individually in each of the plurality of stand-alone trains, could be combined and carried out jointly in order to reduce the capital costs involved in building and operating an LNG plant. It would also be desirable to be able to utilize the heat-exchange capability of the boil-off gases from LNG storage tanks to improve the overall efficiency of the gas liquefying process.
The present invention provides natural gas liquefaction systems and processes wherein certain components of the process equipment normally found in each stand-alone train of an LNG plant are eliminated from the trains. As used hereinafter, the term xe2x80x9cdependent trainxe2x80x9d includes any unit in an LNG plant that lacks one or more of the following components: a flash tank, a reject gas heat exchanger, or a fuel gas compressor. A common flash tank, a common reject gas heat exchanger, and a common fuel gas compressor are positioned in the storage area near the LNG storage tanks which, in turn, are located a substantial distance (e.g., at least about 1 kilometer) from the dependent trains in the plant area. Each common component carries out its respective function for all of the dependent trains. An advantage of this invention is that boil-off gas from the storage tanks may be used for cooling in addition to being used as a fuel gas, as is further explained in the following. In some embodiments of this invention, the distance between the LNG storage tanks and the dependent trains may be shorter than 1 kilometer.
More specifically, the present invention relates to a system for liquefying natural gas having a plurality of dependent trains, each of which comprises a cryogenic heat exchanger system. That is, each dependent train receives the feed gas, i.e. natural gas, and cools it to cryogenic temperatures. The cooled feed gas from the plurality of dependent trains is combined and flowed to the storage area where it is flashed through a common flash valve or common hydraulic turbine for pressure reduction and is then flowed into a common flash tank where it separates into LNG and a vapor (i.e. reject gas). As used herein, the term xe2x80x9ccommon flash devicexe2x80x9d refers to either a common flash valve or a common hydraulic turbine.
The LNG is flowed to a storage tank while the reject gas is flowed through and warmed in a common reject gas heat exchanger. The warmed reject gas is then flowed on to a common fuel gas compressor to increase its pressure before being used as a fuel gas. The boil-off gas from the storage tank(s) is also flowed through the common reject gas heat exchanger where it is warmed before it is combined with the reject gas and fed to the common fuel gas compressor. Both the reject gas and the boil-off gas are heat exchanged with a fluid stream which, in turn, is cooled to a cryogenic temperature and flowed to the common flash tank. The fluid stream may comprise a portion of the feed gas, scrub column overhead gas, and/or some other fluid(s).
It can be seen that the amount of equipment required for liquefying the natural gas is reduced and that the cooling capacity of the boil-off gas from the storage tanks is utilized, thereby increasing the overall efficiency of the process, as compared to a typical LNG plant comprised of only stand-alone trains.